Wet gas purification method and system for practicing the same

ABSTRACT

The present invention provides a wet gas purification method in which ammonia in the gas is removed, and a system for carrying out said method, characterized in that the method includes a washing step in which ammonia in the gas is absorbed in an absorbent for removal; and an ammonia treating step in which ammonia is stripped from the discharged absorbent at a following stage of the washing step, wherein the absorbent is divided into an off-gas containing ammonia and effluent. In the washing step makeup water is charged continuously or intermittently so that the concentration of ammonia in the gas having passed through the washing step is 10 ppm or lower. According to the present invention, a wet gas purification method can be performed in which the running costs involved in operation is reduced, the manipulation step and system are simple, operation is easy, and the reliability is high.

TECHNICAL FIELD

The present invention relates to a wet gas purification method forremoving ammonia from a gas. More particularly, the present inventionrelates to a wet gas purification method and a wet gas purificationsystem suitable for removing ammonia from a gas such as a coal or heavyoil gasification gas.

RELATED ART

Conventionally, in a wet purification system for a coal or heavy oilgasification gas, sulfuric acid is added upstream of an H₂S absorptiontower as a pH adjuster. This allows for the removal of NH₃, which is asource of fuel NOx in gas turbines, during the water-washing step forremoving impurities in the gas.

Specifically, if a gas containing ammonia is burned in a gas turbinewithout being purified, the gas becomes a NOx source, so that it isnecessary to recover as much ammonia as possible. To absorb the ammonia,a method for decreasing the pH of an absorbent in a water-washing toweris carried out, by which most of the ammonia can be removed. As a pHadjuster, sulfuric acid is preferably used in adjusting alkali ofammonia. The pH of the water-washing tower can be reduced by theaddition of the sulfuric acid.

The ammonia removed in the above-described water-washing step is takenout by an ammonia stripper. Since sulfuric acid is added to theabsorbent, ammonium sulfate exists in the discharged solution from theammonia stripper. The ammonia is recovered as aqueous ammonium sulfate.2NH₃+H₂SO₄→(NH₄)₂SO₄

To expel ammonia components from the aqueous ammonium sulfate and torecover ammonia as aqueous ammonia by a stripping method, caustic soda(NaOH) is added before the stripping. Usually, the caustic soda ischarged into a neutralization tank provided upstream of the stripper andis mixed. Then, the mixed liquid is sent to the stripper. The additionof NaOH yields sulfuric acid and ammonia again as described in thefollowing formula. Then ammonia is recovered.(NH₄)₂SO₄ →2NH₃+H₂SO₄

On the other hand, in the above-described conventional system, one ofmajor reasons for adding sulfuric acid is to facilitate the operationcontrol of the system. That is to say, if the pH value of the liquid isdecreased by sulfuric acid, ammonia can be recovered and removed. Thereis no need for monitoring and controlling the amount of ammonia at thefollowing step of the water-washing step, and the control of pH sufficesfor operation. Because of such ease of operation, a method in whichsulfuric acid is added has been used.

Also, by decreasing the pH value, hydrogen sulfide is not removed in thewater-washing step, but passes through easily. A higher pH valuepresents a disadvantage of dissolution of hydrogen sulfide in theliquid, so that the treatment of effluents is complicated. Therefore, inthe conventional step, the separation/removal step of ammonia and theremoval step of hydrogen sulfide are separated by changing the pH valueso that in the ammonia separation step, hydrogen sulfide does notdissolve in the liquid, and is not removed from the gas.

However, in such a conventional step, a step for adding sulfuric acidand caustic soda is needed, so that the operation cost increases due tothe addition of these chemicals. Specifically, in order to treat 1000ppm of ammonia existing in the gas, ½ mole of sulfuric acid is neededfor every 1 mole of ammonia, which increases the chemical cost. Also, inorder to treat the nitrogen components in the effluent from awater-washing tower, which contains absorbed ammonia, at least 2 molesof caustic soda (NAOH) are required per 1 mole of sulfuric acid.Further, an amount of caustic soda equivalent to the amount of ammoniais needed. Therefore, the chemical expense is great. Thus, while theammonia can be recovered, the after-treatment steps are complicated andincrease in number.

Further, since the recovered aqueous ammonia accounts for about 20percent by weight, there is no other choice but to treat it as waste,which poses a problem of treatment cost.

DISCLOSURE OF THE INVENTION

In view of the above problems, the inventors carried out studies todevelop a method for purifying an ammonia-containing gas in whichchemicals such as sulfuric acid and caustic soda, which have beenregarded as necessary, are not used at all, by which the running costinvolved in operation is reduced, emissions are restrained effectively,the manipulation step and system are simple and the operation is easy,and the reliability is high.

As a result, the inventors arrived at the present invention throughtheir discovery that the above problems can be solved by continuouslymeasuring the concentration of ammonia in the gas after the washingstep, and charging makeup water in the washing step in place of theconventional addition of sulfuric acid so that the concentration is 10ppm or lower.

Specifically, the present invention provides a wet gas purificationmethod for removing ammonia in the gas comprising: a water-washing stepfor absorbing ammonia in the gas into absorbent to remove the ammonia;and an ammonia treating step for stripping the ammonia from theabsorbent to be discharged after the water-washing step to separate anoff-gas containing ammonia from effluents, wherein, makeup water ischarged continuously or intermittently in the water-washing step so thatthe concentration of the ammonia is 10 ppm or lower after thewater-washing step. The washing step may comprise a cooling step and acleaning step. That is to say, the partial pressure of NH₃can bedecreased by decreasing the concentration of ammonia in aqueoussolution, rather than by actively controlling the pH value, to keep theNH₃ absorbing performance.

In the present invention, it is preferable that the method furthercomprises a step for burning the off-gas. By this off-gas burning step,hydrogen sulfide dissolved in the water-washing step can also bedecomposed simultaneously with the decomposition of ammonia.

Further, in the present invention, it is preferable that effluents inthe ammonia treating step be circulated charged to the washing step asmakeup water.

In the present invention, even if hydrogen sulfide is dissolved in theabsorbent, it can be separated to be removed from the liquid by astripper, so that there is no need for treating hydrogen sulfide in theliquid. The off-gas containing ammonia and hydrogen sulfide can betreated by being burned in the combustion furnace at a following step.It is efficient for the total system to burn the off-gas using aregenerative combustion furnace or a direct burning combustion furnacefor burning the off-gas coming out of a hydrogen sulfide absorbing step.According to the present invention, charging only makeup water can makeammonia stripping easy. Further, the discharged substance can be easilyused for combustion treatment.

According to the present invention, since only makeup water is charged,and neither sulfuric acid nor caustic soda is used, the cost requiredfor operation can be reduced significantly, and also the manipulationstep can be made simple and easy.

Since the makeup water is added, ammonia can exist in the form ofammonium carbonate (NH₄)₂CO₃ by the action of existing carbon dioxide.When the accumulated (NH₄)₂CO₃ is heated and the temperature rises, the(NH₄)₂CO₃ can release carbon dioxide to yield ammonia again. Bystripping ammonia NH₃ and carbon dioxide (CO₂) without using NaOH inthis manner, the cost required for operation can be reduced. Also, byburning H₂S and NH₃ in the effluent stripping gas, the cost for treatingaqueous ammonia can be eliminated.

The present invention is explained in detail below with reference to anembodiment. The scope of the present invention is not limited by thisembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the outline of a system suitable fora wet gas purification method in accordance with the present invention;

FIG. 2 is a schematic view showing a configuration of a purificationsystem used in Example 1; and

FIG. 3 is a graph in which the concentration of ammonia in a gasdischarged from the outlet of a cleaning tower is plotted against theamount of makeup water.

In the above figures, reference numeral 1 denotes a gas cooling tower, 2denotes a gas cleaning tower, 3 denotes a flash drum, 4 denotes anammonia stripper, 5 denotes an off-gas fired furnace, and 7 denotes acirculating pump.

BEST MODE FOR CARRYING OUT THE INVENTION

A specific embodiment of a wet gas purification method in accordancewith the present invention will now be described with reference to theattached drawings.

FIG. 1 is a schematic view showing one example of a system suitable fora wet gas purification method in accordance with the present invention.In the system of the embodiment, a water-washing step may comprise acooling step and a cleaning step. The cooling step may be performed by agas cooling tower 1, and the cleaning step may be performed by a gascleaning tower 2. Makeup water for absorbing ammonia components in a gasis continuously or intermittently charged into the gas cleaning tower 2.By these two water-washing towers, ammonia in the gas can be absorbed inan absorbent, and thereby removed. The amount of the makeup water to becharged is controlled so that the concentration of ammonia in the gas is10 ppm or lower after the water-washing step. The gas discharged fromthe gas cleaning tower 2 is sent to a hydrogen sulfide absorption tower.

In the washing step of the embodiment, ammonia in the gas is removed byadding makeup water without adding sulfuric acid for adjusting the pHvalue. The washing step (carried in the water-washing tower) consists oftwo steps, which use two towers: gas cooling tower 1 and the gascleaning tower 2. The makeup water charged into the gas cleaning tower 2can be circulated by a pump 7 and acts as the absorbent for absorbingammonia. Some of the absorbent is sent into the gas cooling tower 1provided upstream of the gas flow direction, and is circulated by a pump7 in the gas cooling tower 1 as well.

Conventionally, in the gas cooling tower 1, a step of adding sulfuricacid has been carried out. In the embodiment, makeup water is introducedinto the gas cleaning tower 2 in place of sulfuric acid. Thereby, theconcentration of ammonia in the absorbent in the whole of the washingstep is decreased, and ammonia is removed. Specifically, when theconcentration of ammonia in the absorbent becomes high, the absorbenttends to release ammonia due to partial pressure. Thus, it becomesdifficult to remove ammonia using the absorbent in view of reactionequilibrium. Therefore, supplying makeup water continuously orintermittently can keep absorbent in a state of being able to absorb andremove ammonia.

Supplying makeup water in such a way allows the ammonia to react withcarbon dioxide in the gas to dissolve in the absorbent as ammoniumcarbonate. Ammonia is more easily stripped in this state, since the pHvalue of the absorbent is high compared with when sulfuric acid is alsopresent. Thus, ammonia and carbon dioxide can be taken out of theabsorbent by raising the temperature of an ammonia stripper provideddownstream of the water-washing towers without requiring the addition ofcaustic soda.

In the system the absorbent may be water (H₂O). The circulation of theabsorbent in the water-washing towers by the operation of the systemresults in the increased concentration of ammonia in the absorbent. Asdescribed above, in the embodiment, a gas cooling tower 1 and gascleaning tower 2 can be provided. By these two-towers, the concentrationof ammonia can be made low at the outlet of the gas cooling tower 1.Thus, water containing less dissolved components can always becirculated in the gas cleaning tower 2, so that ammonia can be absorbedto be removed effectively.

However, the washing step need not necessarily be provided with twotowers as in the embodiment. A water-washing tower consisting ofone-tower can also be used. Inside of the one tower can be divided intotwo, and gas cooling and gas cleaning can be performed in the one tower.In the tower, liquids at the upper part and lower part which can bedivided by a tray etc. are circulated separately.

Charging makeup water is carried out so that the concentration ofammonia in the charged yielded gas is successively monitored and anamount of makeup water best suitable for the amount of ammonia each timecan be charged.

The monitoring of the amount of ammonia can be performed, for example,by measuring the concentration thereof in the yielded gas just beforethe gas cooling tower 1.

On the other hand, ammonia in the gas, which is charged into, forexample, a gas turbine must be reduced as far as possible because theammonia is responsible for fuel NOx generation. For example, when theconcentration of ammonia is reduced to 10 ppm or lower, the amount ofcharged makeup water is controlled to achieve the concentration.Therefore, it is usually preferable that the operation be performed bymonitoring the concentration of ammonia in the gas near the outlet ofthe gas cleaning tower 2 at the later stage of water-washing step and bychecking that the concentration of ammonia has a value not higher than afixed value.

For the absorbent discharged from the water-washing step, some of theabsorbent drawn from the absorbent circulating in the gas cooling tower1 is usually sent to the ammonia stripper 4 via a flash drum 3. In thestripper 4, ammonia is stripped from the absorbent in which ammonia hasbeen absorbed. Then, the absorbent is divided into an off-gas containingammonia and the remaining washing liquid. The ammonia stripper 4 isusually operated at about 80° C. at the upper stage and at about 130° C.at the lower stage.

Also, in this stripper 4, hydrogen sulfide components contained in theabsorbent are also removed, and are contained in the off-gas togetherwith ammonia. Therefore, the liquid after stripping does not containhydrogen sulfide. Such an off-gas containing ammonia and a minute amountof hydrogen sulfide is sent into an off-gas fired furnace 5, whereby theammonia and hydrogen sulfide are simulaneously burnt.

According to the above-described embodiment, the amount of effluent isincreased by the charge of makeup water. Therefore, it is preferablethat the effluent in the ammonia treating step be used by circulation asthe makeup water charged in the washing step.

A system in which the water coming out of the stripper 4 is used againas makeup water is advantageous because the amount of water treatedfinally as effluent does not increase as in the conventional example.Also, the system is preferable in terms of the ease with which makeupwater can be secured. Since the water discharged from the stripper 4usually has a temperature increased to 100° C. or higher, the water maybe supplied into the gas cleaning tower 2 as makeup water after beingcooled.

The gas to be purified in the present invention may be any gascontaining ammonia. Specifically, for example, the gas includes, but isnot limited to, coal gasification gas containing much ammonia andhydrogen sulfide.

The gas purification method in accordance with the present invention cansuitably used as a wet purification method for coal gasification gas atthe preceding step of a hydrogen sulfide removing step as part of asystem in which coal is gasified and used as power generation fuel. Bythe use of the above-described purification method in accordance withthe present invention, purification can be performed very efficiently inthe system for treating ammonia-containing gas.

According to the treating method in accordance with the presentinvention, since neither sulfuric acid nor caustic soda is used, thecost required for operation can be reduced significantly. Also, sinceammonia is accumulated as (NH₄) ₂CO₃, NH₃and CO₂ are stripped byincreasing the temperature without the use of NaOH, by which the costrequired for operation can be reduced.

On the other hand, HCN components, which are other harmful componentscontained in the gas, cannot be absorbed and removed unless theabsorbent has a high pH value. HCN components are scarcely removed by anabsorbent having a pH value of about 5 to 6. In the conventional methodin which sulfuric acid is added, the pH value of absorbent decreases (toabout pH 5 to 6). By contrast, in the present invention, the pH value ofabsorbent in the water-washing towers increases (to about pH 6 to 9).Therefore, according to the purification method in accordance with thepresent invention, the HCN removing performance is improved by theincrease in pH value of absorbent to about 6 to 9.

Also, according to the present invention, causes of material corrosioncan be reduced by the increase in pH value of absorbent in thewater-washing towers and the decrease in concentration of chlorine inthe absorbent due to the addition of makeup water.

The experimental results showing the advantageous effects of the presentinvention will now be explained in detail as an example. However, thepresent invention is not limited by the example.

EXAMPLE 1

FIG. 2 shows the outline of a system in example 1.

Two towers are provided: the gas cooling tower 1 is provided upstream ofthe flow of gas, and the gas cleaning tower 2 is provided downstream ofthe flow of gas. From the inlet of the gas cooling tower 1, a yieldedgas having an ammonia concentration (y0) of 1200 ppm was introduced at agas flow rate of 3500 m³N/h under a pressure of 0.9 MPa.

The amount (× kg/h) of makeup water charged into a circulation line ofthe gas cleaning tower 2 was changed continuously in the range of 40 to350 kg/h.

When the temperatures of both of the gas cooling tower 1 and the gascleaning tower 2 were set at 40° C., as shown in FIG. 3, the ammoniaconcentration (y2) measured at the outlet of the gas cleaning tower 2decreased as the amount of makeup water increased. The amount of makeupwater in this example indicates the amount in a preliminary plant. Fromthe results, it was found out that if the amount of makeup water chargedin the washing step is increased the concentration of ammonia in theyielded gas can be decreased

1. A wet gas purification method for removing ammonia in a gascomprising: water-washing step for absorbing ammonia in the gas intoabsorbent to remove the ammonia; and an ammonia treating step forstripping the ammonia from the absorbent discharged after thewater-washing step to separate an off-gas containing ammonia fromeffluent, wherein, makeup water is charged continuously orintermittently in the water-washing step so that the concentration ofthe ammonia is 10 ppm or lower after the water-washing step.
 2. The wetgas purification method according to claim 1, wherein the water-washingstep comprises a cooling step and a cleaning step.
 3. The wet gaspurification method according to claim 1, further comprising a step ofburning the off-gas after the ammonia treating step.
 4. The wet gaspurification method according to claim 1, wherein the effluent in theammonia treating step is circulated to the washing step as makeup water.5. A wet gas purification system for removing ammonia in the gas,comprising: a water-washing tower for absorbing ammonia in the gas intoabsorbent to remove the ammonia; and an ammonia stripper for strippingthe ammonia from the absorbent discharged downstream of thewater-washing tower to separate an off-gas containing ammonia fromeffluent wherein, makeup water is charged continuously or intermittentlyin the water-washing tower so that the concentration of ammonia is 10ppm or lower on the downstream of the water-washing tower.
 6. The wetgas purification system according to claim 5, wherein the water-washingtower comprises a gas cooling tower and a gas cleaning tower.
 7. The wetgas purification system according to claim 5, further comprising anoff-gas fired furnace for burning the off-gas on the downstream of theammonia stripper.
 8. The wet gas purification system according to claim5, wherein the effluents from the ammonia stripper are circulated to thewater-washing tower as makeup water.